The present invention relates to a variable displacement compressor including a displacement control valve and, more particularly, to a control circuit for controlling operation of the displacement control valve.
Such a variable displacement compressor is filed as Japanese Patent Application No. 9-260345 on Sep. 25, 1997 and disclosed as Japanese Patent Laid-Open No. 11-93832 on Apr. 6, 1999. The variable displacement compressor includes a plurality of pistons and a crank mechanism for driving the pistons in a crankcase. When the pistons are driven through the crank mechanism, a refrigerant gas is moved from a suction chamber to a discharge chamber with being compressed by each of the pistons.
The variable displacement compressor has a communication passage communicating the discharge chamber with the crank chamber and further includes a displacement control valve for opening and closing the communication passage.
Such a displacement control valve is filed as Japanese Patent Application No. 9-271516 on Oct. 3, 1997 and disclosed as Japanese Patent Laid-open No. 11-107929 on Apr. 20, 1999. The displacement control valve comprises a valve element placed in the communication passage and an electromagnetic coil or solenoid which is for driving the valve element by the use of an electric current applied from a power source thereto. When the valve element is driven to open the communication passage, the refrigerant gas is introduced from the discharge chamber into the crank chamber. When the valve element is driven to close the communication passage, the refrigerant gas is prohibited the introduction from the discharge chamber into the crank chamber. Thus, the displacement control valve controls the differential pressure between the crank chamber and the suction chamber. Therefore, the variable displacement compressor has a variable displacement which is controlled in response to the differential pressure.
The electromagnetic solenoid may be duty controlled by a control circuit in the manner known in the art. In this event, the displacement control valve can be controlled in a stable manner when the power source has a voltage which is stable.
It is assumed as a particular case that the voltage of the power source is unstable. In the particular case, it is not possible to control the displacement control valve in a stable manner. As a result, it becomes not possible to achieve stable control of the discharge capacity or displacement of the variable displacement compressor.
It is an object of the present invention to provide control circuit for controlling a current in an electromagnetic coil with a duty ratio which is adjusted in response to variation of a power source voltage.
It is another object of the present invention to provide a control circuit for a displacement control valve included in a variable displacement compressor, which can control operation of the displacement control valve in a stable fashion even when a power source has an unstable voltage.
Other objects of the present invention will become clear as the description proceeds.
According to the present invention, there is provided a control circuit for controlling a coil current flowing in an electromagnetic coil connected to a power source. The electromagnetic coil is included in a displacement control valve for varying a displacement of a variable displacement compressor. The control circuit comprises a current processing circuit connected to the power source for processing the coil current into a pulse current with a duty ratio and a ratio adjusting circuit connected to the power source and the current processing circuit for adjusting the duty ratio in response to variation of a voltage of the power source.